The present invention relates generally to estimating the position of a mobile communications device based on the arrival time of signals broadcast by a plurality of base transceiver stations.
In a cellular radio telecommunications network, it is desirable in many circumstances to be able to at least approximately identify the position of a mobile station (MS) to within a small area. For example, the approximate position of the mobile station may be useful for dispatching an emergency unit to the mobile station site when an emergency call is made from the mobile station. One of the methods of estimating the approximate position of the mobile station uses the time of arrival at the mobile station of the synchronization sequences transmitted at regular intervals by at least three base transceiver stations (BTSs), or base stations, of the cellular network. These base stations include the serving base station of the mobile station and the neighboring base stations. The measured times of arrival (TOAs), are relative to the internal clock of the mobile station, which is synchronized with the synchronization sequences transmitted by the serving base station and user and signaling data transmitted between the mobile station and the cellular network via the serving base station. Based on the TOAs, the Observed Time Differences (OTDs) are calculated. The OTDs represent the time difference of arrival of signals between the serving BTS and the neighboring BTSs. Assuming that the neighboring base stations and the serving base station transmit their respective synchronization sequences at the same time, then the measured OTDs for the neighboring base stations represent the propagation times between the respective neighboring base stations and the mobile station. In practice, the neighboring base stations are not synchronized with the serving base station. Instead, a timing offset or a Real Time Difference (RTD) is used to calculate the geometric time difference (GTD) from the OTD. The RTD regarding a neighboring base station is defined as the difference between the transmission time from that neighboring base station and the transmission time from the serving base station. The RTD is known to a serving mobile location center (SMLC) of the telecommunications network. As disclosed in EP 0 936 758 A2, given the GTDs from a number of neighboring BTSs relative to the serving BTS is known, the approximate position of the mobile station can be obtained from intersecting hyperbolas. This method is known as E-OTD, or Enhanced OTD, in a TDMA system.
The RTD values can be sent to the mobile station to enhance the measurement process. By knowing the RTD value, the mobile station can in theory align the reception window, and take only samples from the point of signal where the training sequence (of normal or dummy burst) is expected to be located. This means that the measurement of frequency correction channel (FCCH) and synchronization channel (SCH) bursts can be avoided, and thus the OTD measurement process is much faster.
The above-described method is useful when the uncertainty of the position of the mobile station is sufficiently small and that is known. In this situation, the mobile station aligns the reception window used for OTD measurements by utilizing the RTDs provided by the network in the E-OTD measurement command. A major setback for this method is that the uncertainty of the position of the mobile station may cause the mobile station not to use the RTDs. It is possible that the OTD to be measured in the current position of the mobile station differs from the reported RTD by several bit periods and even up to tens of bit periods. That makes the RTD values unusable, because the correlation properties of normal and dummy bursts are so poor that this kind of uncertainty is not allowed. Instead, the mobile station needs to first measure the frequency correction channel (FCCH) and synchronization channel (SCH) burst.
Usually, when the uncertainty of the position of the mobile station is small (e.g., comparable to 2 bits), this uncertainty can be tolerated in the OTD measurements. This is particularly true when the cell size is small such as that in an urban environment. In such an environment, if the MS knows that the uncertainty is so small (i.e. cells are small), the RTD value would very likely be useful in the vast majority of the cases. However, because the location of the mobile station is unknown, and thus the magnitude of the uncertainty is also unknown, it is possible that the provided RTD values would not be used by the mobile station at all.
Thus, it is advantageous and desirable to provide a reliable method for estimating the location of the mobile station, and thus the uncertainty of the arrival times from neighboring BTSs.
The present invention makes use of the uncertainty of the arrival times from respective neighboring base stations, caused by the uncertainty of the location of the mobile station, to allow the mobile station to calculate a suitable reception window and select suitable bursts for arrival time measurements based on values provided by the network.
According to the first aspect of the present invention, a method of estimating a location of a mobile station in a mobile telecommunications network having a first base station and a plurality of second base stations neighboring the first base station for providing communication links to the mobile station, wherein the first base station is located at a first site and second base stations are located at respective second sites, and the first and second base stations provide synchronization channel bursts and further bursts to the mobile station for allowing the mobile station to make arrival time measurements. The method comprises the steps of:
providing the mobile station a first value regarding transmission of signals from the respective second base stations to the mobile station;
calculating a second value representative of uncertainty in said arrival time measurements based on the first value and optionally a distance between the mobile station and the first site; and
providing the mobile station a predetermined value such that the mobile station makes said arrival time measurements based on the synchronization channel bursts when the second value is greater than the predetermined value, and the mobile station carries out a time adjustment process when the second value is smaller than or equal to the predetermined value.
Preferably, the first base station is a serving station and the first value is indicative of an expected arrival time of the signals from the respective second base stations to the mobile station as if the mobile station is located at the first site, and the second value is calculated by the mobile station.
Alternatively, the first value is indicative of the distances between the first site and the respective second sites, and the second value is calculated by the mobile station.
Alternatively, the first value is indicative of an expected arrival time of signals from the respective second base stations to the current location of the mobile, as estimated by the serving mobile location center, and the second value is calculated by the serving mobile location center and provided to the mobile station. Furthermore, the second value may also be calculated based on a cell identity of the first base station, antenna sector information regarding the location of the mobile station, and the received signal levels from the first and second base stations as reported by the mobile station.
Preferably, the distance from the first site to the mobile station is provided to the mobile station by the network.
Preferably, the mobile station makes the arrival time measurements based on the further bursts in the time adjustment process when the second value is smaller than or equal to the predetermined value.
Alternatively, the mobile station aligns a time window for taking samples in the signals transmitted from the respective second base stations in the time adjustment process utilizing the second value when the second value is smaller than or equal to the predetermined value.
Preferably, the mobile station further makes the arrival time measurements based on the further bursts in addition to making said arrival time measurements based on the synchronization channel bursts when the second value is greater than the predetermined value.
Preferably, the predetermined value is stored in the mobile station, but it is possible that the predetermined value can be sent to the mobile station by the mobile telecommunications network.
According to the second aspect of the present invention, a system for estimating a location of a mobile station in a mobile telecommunications network having a first base station and a plurality of second base stations neighboring the first base station for providing communication links to the mobile station, wherein the first base station is located at a first site and second base stations are located at respective second sites, and the first and second base stations provide synchronization channel bursts and further bursts to the mobile station for allowing the mobile station to make arrival time measurements. The system comprises:
a first means for providing the mobile station a first value regarding transmission of signals from the respective second base stations to the mobile station;
a second means, responsive to the first value, for providing a second value representative of the uncertainty in said arrival time measurements based on the first value and optionally the distance between the mobile station and the first site; and
a third means, responsive to the second value and a predetermined value, for making said arrival time measurements based on the synchronization channel bursts when the second value is greater than the predetermined value, and the mobile station carries out a time adjustment process when the second value is smaller than or equal to the predetermined value.
Preferably, the first base station is a serving base station and the first value is indicative of an expected arrival time of the signals from the respective second base stations to the mobile station as if the mobile station is located at the first site, and the second means is located in the mobile station for calculating the second value.
Alternatively, the first base station is a serving base station and the first value is indicative of a distance between the first site and a respective second site, and the second means is located in the mobile station for calculating the second value.
Alternatively, the first value is indicative of an expected arrival time of signals from the respective second base stations to the current location of the mobile station, as estimated by a serving mobile location center, and the second means is located in the serving mobile location center for calculating the second value and providing a signal indicative of the second value to the mobile station.
Preferably, the mobile station makes said arrival time based on the further bursts in the time adjustment process when the second value is smaller than or equal to the predetermined value.
Alternatively, the mobile station aligns a time window for taking samples in the transmitted signals from the respective second base stations in the time adjustment process when the second value is smaller than or equal to the predetermined value.
Preferably, the predetermined value is stored in the mobile station, but the predetermined value can also be provided to the mobile station by the mobile telecommunications network.
The present invention will become apparent upon reading the description taken in conjunction with FIGS. 1 to 3.